1. Field of the Invention
This invention generally relates to silicon-on-insulator films and, more particularly, to a method for thinning an SOI film having improved thickness uniformity.
2. Discussion of the Related Art
It has been demonstrated by 2D simulations and experiments that thinning SOI films to less than the maximum depletion depth in the channel region (that is, approximately 1000.ANG. or less) brings about remarkable device properties as described in M. Yoshimi, et al., "Electrical Properties and Technological Perspectives of Thin-Film SOI MOSFETs", IEICE Trans., Vol. E74, No. 2, Feb. 1991, pp. 337-351. In ultra-thin SOI, the device body becomes fully depleted which results in greatly improved short channel characteristics at much lower channel dopings. This also increases the device turn-on voltage, i.e., threshold voltage (V.sub.T), tolerance to body doping and channel length. In addition, full depletion produces near ideal subthreshold slope, nearly 45% improved transconductance and much higher current drivability. This in conjunction with reduced junction capacitances yields greater switching speed with thin SOI CMOS than in comparable bulk Si CMOS. These properties of ultra-thin SOI CMOS open superior possibilities for scaling CMOS structures in the deep sub-half .mu.m regions that are impossible with conventional bulk silicon.
SOI films with thickness near 1000.ANG. can be made using SIMOX (Separation by IMplantation of OXygen) technology. However, SIMOX/SOI wafers suffer from high defect density (&gt;1000 per cm.sup.2). Also, the cost of commercially available thin SIMOX/SOI is very high. On the other hand, wafer bonding and polishing produces defect-free SOI films at much lower cost. However, the minimum SOI thickness achievable with the present commercial wafer bonding technology is about 2 .mu.m with thickness uniformity of about .+-.3000.ANG.. Therefore, thinning the commercially available bonded SOI wafers to the thickness required for fully depleted CMOS (i.e., &lt;1000.ANG.) is an attractive approach for producing ultra-thin near-zero defect SOI material. A method for thinning SOI films having excellent thickness uniformity is highly desirable. Furthermore, a method for producing ultra-thin active device regions having self-aligned isolation regions is highly desirable.